1. Field of the Invention
The present invention relates to a system for steering rear wheels of an automotive vehicle and, more particularly, to a system for steering rear wheels so adapted as to steer front wheels as well as rear wheels.
2. Description of the Related Art
Among systems for steering rear wheels, there is known of such a type as electrically steering the rear wheels completely without mechanical interconnections between a mechanism for steering the rear wheels and a mechanism for steering the front wheels, as disclosed in Japanese Patent Laid-open Publication (kokai) No. 57-44,568. Such an electrically interconnecting type of the system for steering the rear wheels can offer the advantage that the rear wheels can be steered even if the front wheels are sustained in the neutral position, however, it suffers from the disadvantage that it is difficult to set the rear wheels strictly in the neutral position as the front wheels are sustained in the neutral position. Further, such a system may be likely to provide the operator with feeling of uneasiness when the rear wheels are steered in a careless manner in a state in which the front wheels are sustained in the neutral position. In addition, the system has the defect that a sufficient extent of fail measures should be taken at the time of failure.
The above-specified prior patent publication discloses steering characteristics, including a characteristic term for a yaw rate that functions as acting in the direction of converging the yaw rate, if occurred, that is, that functions as an ingredient for correcting a same phase amount in the direction of increasing.
On the other hand, Japanese Patent Laid-open Publication (kokai) No. 62-227,871 discloses a type of mechanically interconnecting a mechanism for steering the rear wheels with a mechanism for steering the front wheels and a steering-ratio altering mechanism is incorporated into such a mechanically interconnecting mechanism. This mechanically interconnecting mechanism can offer the advantage that the rear wheels can be sustained strictly in the neutral position when the front wheels are sustained in the neutral position, however, it suffers from the disadvantage that the rear wheels cannot be steered as long as the front wheels are to be steered.
It is to be noted herein that the rear wheel steering system of the electrically interconnecting type having the characteristic term for the yaw rate as the steering characteristics can negate the yaw rate by steering the rear wheels upon occurrence of the yaw rate due to a side wind during running straight at a high speed, thereby improving stability in running straight.
It is to be noted, however, that the rear wheel steering system of the mechanically interconnecting type having the characteristic term for the yaw rate sustains the rear wheels in the neutral position as long as the front wheels are held in the neutral position, so that the characteristic term for the yaw rate does not substantially serve as improving stability in running straight as long as the vehicle is running straight because the rear wheels cannot be steered even if the yaw rate would be caused to occur due to a side wind.